U.S. patent number 6,613,250 [Application Number 10/116,259] was granted by the patent office on 2003-09-02 for vegetable oil based dielectric fluid and methods of using same.
This patent grant is currently assigned to Cooper Industries, Inc.. Invention is credited to Jerry L. Corkran, Gary A. Gauger, Richard A. Harthun, C. Patrick McShane, Kevin J. Rapp.
United States Patent |
6,613,250 |
McShane , et al. |
September 2, 2003 |
**Please see images for:
( Certificate of Correction ) ** |
Vegetable oil based dielectric fluid and methods of using same
Abstract
In one aspect, the present invention provides a dielectric fluid
for use in electrical equipment comprising a vegetable oil or
vegetable oil blend. In another aspect the invention provides
devices for generating and distributing electrical energy that
incorporate a dielectric fluid comprising a vegetable oil or
vegetable oil blend. Methods of retrofilling electrical equipment
with vegetable oil based dielectric fluids also are provided.
Inventors: |
McShane; C. Patrick (Waukesha,
WI), Corkran; Jerry L. (Waukesha, WI), Harthun; Richard
A. (Burlington, WI), Gauger; Gary A. (Franklin, WI),
Rapp; Kevin J. (Oak Creek, WI) |
Assignee: |
Cooper Industries, Inc.
(Houston, TX)
|
Family
ID: |
27402799 |
Appl.
No.: |
10/116,259 |
Filed: |
April 4, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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288877 |
Apr 9, 1999 |
6398986 |
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276191 |
Mar 25, 1999 |
6184459 |
Feb 6, 2001 |
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728261 |
Oct 8, 1996 |
6037537 |
Mar 14, 2000 |
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576372 |
Dec 21, 1995 |
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Current U.S.
Class: |
252/579;
174/17LF; 252/570; 336/58; 336/94 |
Current CPC
Class: |
C09K
5/10 (20130101); H01B 3/20 (20130101); H01F
27/125 (20130101); H01F 27/14 (20130101); H01F
27/321 (20130101) |
Current International
Class: |
C09K
5/10 (20060101); C09K 5/00 (20060101); H01F
27/12 (20060101); H01F 27/10 (20060101); H01B
3/18 (20060101); H01B 3/20 (20060101); H01F
27/14 (20060101); H01F 27/32 (20060101); H01B
003/20 () |
Field of
Search: |
;252/570,579 ;336/58,94
;174/17LF |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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22559/29 |
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Feb 1930 |
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GB |
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52-25298 |
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Feb 1977 |
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JP |
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WO 97/49100 |
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Dec 1997 |
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WO |
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Other References
Clark, "Insulating Materials for Design and Engineering Pracrice,"
1962, pp. 131, 132, 210-213, 216-218, 342, 344, 345, and 383. .
"Contoured Transformer Unveiled," Transmission & Distribution,
p. 42..
|
Primary Examiner: Ogden; Necholus
Attorney, Agent or Firm: Fish & Richardson, P.C.,
P.A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation (and claims the benefit of
priority under 35 USC 120) of U.S. application Ser. No. 09/288,877,
filed Apr. 9, 1999 now U.S. Pat. No. 6,398,986; which is a
continuation-in-part of U.S. application Ser. No. 09/276,191 filed
Mar. 25, 1999, now U.S. Pat. No. 6,184,459 issued Feb. 6, 2001;
which is a divisional of U.S. application Ser. No. 08/728,261 filed
Oct. 8, 1996, now U.S. Pat. No. 6,037,537 issued Mar. 14, 2000;
which is a continuation of U.S. application Ser. No. 08/576,372
filed Dec. 21, 1995 now abandoned.
Claims
What is claimed is:
1. A method of using an electrical device comprising employing in
said device a food grade dielectric fluid consisting essentially of
an oleate modified vegetable oil, and wherein said fluid is
substantially free of chlorinated compounds and has a viscosity
between 2 and 15 cSt at 100.degree. C., and less than about 100 cSt
at 400.degree. C.
2. The method of claim 1, wherein the fluid further comprises
soybean oil.
3. The method of claim 2, wherein the fluid consists essentially of
75% by weight oleate modified vegetable oil and 25% by weight
soybean oil.
4. The method of claim 2, wherein the fluid consists essentially of
50% by weight oleate modified oil and 50% weight soybean oil.
5. The method of claim 1, wherein said dielectric fluid further
comprises an antioxidant compound.
6. The method of claim 5, wherein said antioxidant compound is
selected from the group consisting of: butylated hydroanisole,
butylated hydrotoluene, tertiary butylhydroquinone,
tetrahydrobutrophenone, ascorbyl palmitate, propyl gallate, and
alpha-, beta- or delta-tocopherol.
7. The method of claim 1, wherein said dielectric fluid further
comprises a pour point depressant.
8. The method of claim 1, wherein said dielectric fluid further
comprises a dye or pigment.
9. The method of claim 1, wherein the fluid further comprises less
than 5% by weight of a petroleum-derived mineral oil.
10. The method of claim 1, wherein the fluid further comprises less
than 1% by weight of a petroleum-derived mineral oil.
11. The method of claim 1, wherein said device is an electrical
transformer.
12. The method of claim 1, wherein said device is an electrical
switchgear device.
13. The method of claim 2, wherein said device is an electrical
transmission cable.
14. A method of using an electrical device comprising employing in
said device a food grade dielectric fluid consisting essentially of
soybean oil and at least one second vegetable oil selected from the
group consisting of rapeseed oil and sunflower oil, and wherein
said fluid is substantially free of chlorinated compounds and has a
viscosity between 2 and 15 cSt at 100.degree. C., and less than
about 100 cSt at 40.degree. C.
15. The method of claim 14, wherein the fluid consists essentially
of 25% by weight soybean oil and 75% by weight of one of a second
vegetable oil selected from the group consisting of rapeseed oil
and sunflower oil.
16. The method of claim 15, wherein the second vegetable oil is
rapeseed oil.
17. The method of claim 15, wherein the second fluid is sunflower
oil.
18. The method of claim 14, wherein said dielectric fluid further
comprises an antioxidant compound.
19. The method of claim 18, wherein said antioxidant compound is
selected from the group consisting of: butylated hydroanisole,
butylated hydrotoluene, tertiary butylhydroquinone,
tetrahydrobutrophenone, ascorbyl palmitate, propyl gallate, and
alpha-, beta- or delta-tocopherol.
20. The method of claim 14, wherein said dielectric fluid further
comprises a pour point depressant.
21. The method of claim 14, wherein said dielectric fluid further
comprises a dye or pigment.
22. The method of claim 14, wherein the fluid further comprises
less than 5% by weight of a petroleum-derived mineral oil.
23. The method of claim 14, wherein the fluid further comprises
less than 1% by weight of a petroleum-derived mineral oil.
24. The method of claim 14, wherein said device is an electrical
transformer.
25. The method of claim 14, said device is an electrical switchgear
device.
26. The method of claim 14, wherein said device is an electrical
transmission cable.
Description
FIELD OF THE INVENTION
In one aspect, the present invention relates to dielectric fluid
compositions, including insulating oils, for use in electrical
distribution and power equipment, including transformers, switching
gear, and electric cables. In another aspect the invention relates
to vegetable oil-based insulating fluids and, more particularly, to
the use of compositions comprising one or more vegetable oils. In
yet another aspect, the present invention relates to the
modification of electrical distribution equipment in a manner that
enhances their suitability for vegetable oil-containing dielectric
fluid compositions.
BACKGROUND OF THE INVENTION
Dielectric (or insulating) fluids used in electrical distribution
and power equipment--including transformers, switching gear and
electric cables--perform two important functions. These fluids act
as an electrical insulating medium, i.e., exhibit dielectric
strength, and they transport generated heat away from the
equipment, i.e., act as a cooling medium. When used in a
transformer, for example, dielectric fluids transport heat from the
windings and core of the transformer or connected circuits to
cooling surfaces. Apart from possessing dielectric strength and
cooling capacity, an ideal dielectric fluid for electrical
equipment also exhibits little or no detrimental impact on the
environment, is compatible with materials used to construct the
equipment, and is relatively nonflammable.
For more than a century, mineral oils derived from crude petroleum
were used extensively as insulating and cooling liquids in
electrical equipment. Though such oils possess a satisfactory
dielectric strength and are compatible with equipment materials,
they are not considered nonflammable, and, because they are
petroleum-based, they are considered to carry with them an
environmental cost. In the middle part of this century, as safety
standards became more demanding for many indoor and vault equipment
installations, mineral oils were replaced to a large extent by
nonflammable liquids such as askarel (polychlorinated biphenyl, or
"PCB") fluids. Beginning in the 1930s, for example, PCBs--which
generally are considered nonflammable--were used extensively to
replace mineral oils in fire sensitive locations as insulating
fluids in electrical equipment.
PCBs eventually were recognized for their environmental hazards,
and as a result the production and sale of PCBs as well as their
use in new equipment was banned. For existing PCB-filled equipment,
stringent regulations now require removal of PCB fluids at certain
installations and, for all other installations, place stringent
restrictions on the use of PCB-filled equipment. Spill reporting,
clean-up, and disposal of PCB-filled equipment also now require
compliance with very strict EPA regulations.
Because of the disadvantages and shortcomings of PCB-based fluids
and because of the increasing sensitivity to the potential adverse
environmental impact of mineral oils and available alternatives,
there have been and continue to be numerous efforts undertaken to
develop relatively inexpensive, environmentally safe, and
nonflammable dielectric fluids. To date, these efforts have not
been completely successful.
There are a number of specific functional properties characteristic
of dielectric oils. An oil's dielectric breakdown, or dielectric
strength, for example, provides an indication of its ability to
resist electrical breakdown and is measured as the minimum voltage
required to cause arcing between two electrodes at a specified gap
submerged in the oil. The impulse dielectric breakdown voltage
provides an indication of an oil's ability to resist electrical
breakdown under transient voltage stresses such as lightning and
power surges. The dissipation factor of an oil is a measure of the
dielectric losses in the oil; a low dissipation factor indicates
low dielectric loss and a low concentration of soluble, polar
contaminants. The gassing tendency of an oil measures the oil's
tendency to evolve or absorb gas under conditions where partial
discharge is present.
Because one function of a dielectric fluid is to carry and
dissipate heat, factors that significantly affect the relative
ability of the fluid to function as a dielectric coolant include
viscosity, specific heat, thermal conductivity, and the coefficient
of expansion. The values of these properties, particularly in the
range of operating temperatures for the equipment at full rating,
must be weighed in the selection of suitable dielectric fluids for
specific applications.
In addition to the foregoing properties that affect heat transfer,
a dielectric fluid, to be useful in commercial applications, should
have a relatively high dielectric strength, low dissipation factor,
a dielectric constant that is compatible with the solid dielectric,
a low gassing tendency, and it must be compatible with the
electrical equipment materials to which it is exposed.
Current codes and standards require further that any dielectric
fluid intended for use as a coolant not be classified as
"Flammable," but rather as a Class IIIB Combustible liquid.
Specific safety requirements, however, vary with the application to
which the electric equipment containing the fluid is used. Such
applications include, for example, indoor and rooftop
installations, vault applications, and installations adjacent to
building structures. According to the degree of hazard attendant to
these varied applications, one or more additional safeguards may be
required. One recognized safeguard is the substitution of
conventional mineral oils with "less-flammable" and/or nonflammable
liquids. Less-flammable liquids are considered to be those having
an open-cup fire point equal to or greater than 300.degree. C.
Several dielectric fluids are known and used in electrical
equipment. Due, however, to an increasing awareness and sensitivity
toward environmental concerns, it has become increasingly desirable
to provide a dielectric fluid that: (1) poses minimal environmental
hazards; (2) degrades quickly and easily so that spills do not
contaminate the soil or the water table for any significant period
of time; and (3) does not interfere in any significant way with
natural biodegradation processes. It also is becoming more
desirable to replace non-renewable resources with renewable
resources, particularly given the undesirability of dependence on
petroleum-derived products, and there generally is increased demand
by the industrial and retail markets for all-natural products. This
is due, at least in part, from the attention paid to the long-term
effects of materials and their degradation by-products.
In prior, related co-pending application Ser. No. 08/728,261--which
is incorporated in its entirety by reference--we described a class
of insulating dielectric fluids comprising vegetable oil materials.
These compositions, useful in electrical distribution and power
equipment, utilize low maintenance vegetable oil-based dielectric
coolants that meet or exceed applicable safety and performance
standards and that are free of substantial environmental
hazards.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides a dielectric fluid
for use in electrical equipment. The dielectric fluid comprises a
vegetable oil or vegetable oil blend. In another aspect the
invention provides devices for transforming, generating, and/or
distributing electrical energy, including electrical transmission
cables, switching gear and transformers, that incorporate a
dielectric fluid comprising a vegetable oil or vegetable oil blend.
In yet another aspect, the invention provides methods of
retrofilling electrical equipment with vegetable oil-based
dielectric fluids.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 provides a cross-sectional view of a transformer tank
housing incorporating a vegetable oil-based dielectric fluid and
oxygen absorption material housed in an oxygen permeable
encasement.
FIG. 2 shows oxygen absorption material housed in an oxygen
permeable encasement fastened to the tank cover of an electrical
transformer.
FIGS. 3-4 provide cross-sectional views of a transformer tank
incorporating a vegetable oil-based dielectric fluid and an oxygen
absorption material housed in an oxygen permeable encasement.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As their most essential component, the dielectric fluids of the
present invention comprise one or more vegetable oil compositions,
many of which are derived from plant matter. Vegetable oils
typically comprise mixed glycerides formed from a polyol backbone,
such as glycerin, in which the constituent hydroxyl groups are
esterified with an equal or nearly equal number of fatty acid
molecules. Many useful vegetable oils are triglycerides, i.e., are
glycerides having three fatty acid molecules chemically bonded to
the glycerin backbone. Such triglycerides generally are of the
formula: ##STR1##
wherein R.sub.1, R.sub.2 and R.sub.3 each, independently, is an
alkyl or alkenyl group that may be straight-chained or branched,
may be saturated or unsaturated, and may be unsubstituted or may be
substituted with one or more functional or non-functional
moieties.
Differences in the functional properties of vegetable oils
generally are attributable to the variation in their constituent
fatty acid molecules. Several different fatty acids exist,
including the following, all of which may be present in the
vegetable oils of the invention: myristic, palmitic, stearic,
oleic, linoleic, linolenic, arachidic, eicosenoic, behenic, erucic,
palmitiolic, docosadienoic, lignoseric, tetracossenoic, margaric,
margaroleic, gadoleic, caprylic, capric, lauric, pentadecanoic, and
heptadecanoic acids. These fatty acid molecules can also vary in
their degree of unsaturation.
Fatty acid molecules may be arranged on a polyol backbone in any
number of ways, and each polyol can have one, two or several
different constituent fatty acid molecules. The three fatty acid
molecules on a triglyceride molecule, for example, may be the same
or may comprise two or three different fatty acid molecules. While
the compositions of triglyceride compounds found in plant matter
vary from species to species, and less so from strain to strain of
a particular species, vegetable oil derived from a single strain of
plant species generally will have the same fatty acid
composition.
Every naturally occurring triglyceride has a unique set of
properties. For example, some triglycerides are more susceptible to
oxidation than are others. According to the present invention, it
is preferred to use oils having fatty acid molecules that include a
component having at least one degree of unsaturation (i.e., at
least one C.dbd.C double bond). This selection strikes a balance
between the effects of oxidation with a desired reduction in the
evolution of hydrogen gas. It has been found that oils containing
mono-unsaturates oxidize less rapidly than do polyunsaturated oils
and are therefore somewhat preferred for use in the present
invention. Specific, representative, vegetable oils suitable for
use in the present invention include the following: castor,
coconut, corn, cottonseed, crambie, jojoba, lesquerella, linseed,
olive, palm, rapeseed (canola), safflower, sunflower, soya, and
veronia.
Useful vegetable oils preferably have an open-cup fire point well
above the accepted minimum standard of 300.degree. C. for both
conventional dielectric fluids and for "less-flammable liquids."
Several oils, for example, typically have fire points of
approximately 350.degree. C. According to the present invention,
preferred oils have viscosities between about 2 and about 15 cSt at
100.degree. C. and less than about 110 cSt at 40.degree. C. and
have heat capacities (specific heats) of greater than about 0.3
cat/g-.degree. C. The vegetable oils of the invention also
preferably have a dielectric strength of greater than about 30
kV/100 mil gap, more preferably greater than 35 kV/100 mil gap, and
have a dissipation factor of less than about 0.05% at 25.degree.
C., more preferably less than about 0.03% at 25.degree. C.
The vegetable oils of the invention may be used alone or may be
blended together with one or more other vegetable oils. In
appropriate circumstances, a vegetable oil or vegetable oil blend
may also be combined with a minor amount of one or more synthetic
oils, including mineral oils. When a vegetable oil or vegetable oil
blend is combined with one or more synthetic oils, the amount
and/or character of the non-vegetable oil component of the
resulting blend should not interfere with the beneficial properties
of the vegetable oil fluid. Thus, for example, any significant
amount of a chlorinated fluid (aromatic chlorinated compounds such
as trichlorobenzene or polychlorinated biphenyls) will negate many
of the positive environmental attributes of the vegetable oil
component. Where such blends are employed, the blend should contain
less than 50 percent by weight of a petroleum-derived mineral oil,
preferably less than 30 percent by weight, more preferably less
than 20 percent by weight, and should contain less than 20 percent
by weight of a chlorinated fluid, preferably less than 5 weight
percent, and more preferably less than 1 percent by weight. It is
also preferred that the vegetable oil blend be "food grade," i.e.,
that it not contain any component that is considered toxic or
otherwise biologically hazardous.
The vegetable oil and oil blends, where desired, may be pigmented
or colored with a suitable dye or pigment. Any known dye or pigment
can be used for this purpose, and many are available commercially
as food additives. The most useful dyes and pigments are those that
are oil soluble.
Because of its negative effect on dielectric performance, the
presence of water, a polar contaminant, in the vegetable oil-based
fluid is undesirable. Water in the fluid tends to increase the rate
of chemical breakdown of fatty acid esters in the vegetable oil in
proportion to the amount of water available for such a reaction.
The most obvious indicator of such reactions is a significant
increase in the value of the neutralization number as measured by
ASTM D974.
This problem can be compounded by the wide temperature range over
which electrical distribution equipment must operate. It is known
that the dielectric breakdown characteristics and other dielectric
properties of mineral oils are directly related to the percent
saturation of water present in the oil. The water saturation point
of an oil is in turn a function of temperature. As the saturation
point is reached, dielectric strength falls rapidly. The water
saturation point for mineral oils typically used as dielectric
coolants is approximately 65 ppm at room temperature but over 500
ppm at normal operating temperatures (approx. 100.degree. C.).
Electrical distribution equipment exposed to a wide variation in
temperature can suffer a fluctuation in the degree of water
saturation in the dielectric fluid, and water that is dissolved or
in vapor/liquid equilibrium at high operating temperatures can
precipitate or condense when the temperature of the oil
decreases.
Currently accepted standards typically require the removal of
moisture from conventional mineral oils to below about 35 ppm for
use in new distribution equipment. The moisture removal process
uses either evaporation in a reduced pressure chamber or
filtration, or a combination of both, to reach a level of between
about 15 and 25 percent saturation at room temperature (10-15 ppm)
prior to filling of the distribution equipment.
In contrast to mineral oils, vegetable oils generally have much
higher moisture saturation points; typically over 500 ppm at room
temperature. Therefore, acceptable moisture levels in vegetable
oils used in new distribution equipment can be much higher than
those for conventional mineral oils. Because the presence of water
in vegetable oils can cause the additional breakdown of the
constituent fatty acid esters, however, the moisture removal
process used in the preparation of vegetable oil-based dielectric
fluids should strive for moisture levels that reach below, as a
percentage of saturation, those typically required for mineral
oils. A moisture level between about 5 and about 10 percent of the
saturation level of water in the vegetable oil at room temperature
is preferred. The oils also are preferably processed by filtration
or other suitable means to remove particulate and other
contaminants. This can be accomplished in a manner similar to the
techniques for treating and processing convention mineral oil-based
dielectric materials.
Most useful vegetable oils are susceptible to polymerization upon
exposure to free oxygen. Free oxygen activates unsaturated bonds in
such vegetable oils to begin an oxidative polymerization process.
Such polymerization manifests itself in a marked increase in
viscosity and a corresponding decrease in dielectric properties of
the affected oil. The rate of this polymerization is, in part, a
function of the temperature of the oil at the time of exposure to
free oxygen, and the by-products produced as a result of such
polymerization are undesirable because they have chemical
properties that are inferior to the virgin, or unpolymerized, oils.
This degradation of vegetable oil by oxidative polymerization is
due to long-term exposure to free oxygen, and it therefore can
escape immediate detection.
The dielectric fluids of the invention optionally further comprise
an oxidation reducing composition. Such compositions comprise one
or more compounds that absorb, or scavenge, oxygen that otherwise
would dissolve in the vegetable oil composition and result in
oxidative breakdown of the oil. When used, the oxygen absorbing
compound is preferably encased in a housing composed primarily of a
polymeric material that is substantially permeable to oxygen and
substantially impermeable to water and water vapor and that
exhibits a high degree of mechanical strength throughout the
operating temperatures of the electrical equipment in which they
are employed.
Useful oxidation reducing compounds are those that are capable of
reducing the concentration of free oxygen in the atmosphere
surrounding the dielectric fluid inside the sealed housing of
electrical distribution equipment and that in turn reduce the
presence of dissolved oxygen in the fluid itself. Such compounds
can be referred to as oxygen scavenging compounds. Useful oxygen
scavenging compounds include those commonly employed in the food
packaging industry. Representative of the oxygen scavenging
compounds useful in the practice of the invention include the
following: sodium sulfite; copper sulfate pentahydrate; a
combination of carbon and activated iron powder; mixtures of
hydrosulfite, calcium hydroxide, sodium bicarbonate and activated
carbon; a metal halide powder coated on the surface of a metal
powder; and combinations of alkali compounds, such as calcium
hydroxide, with sodium carbonate or sodium bicarbonate. Mixtures
and combinations of one or more of the above compositions are also
considered useful.
Also useful as oxygen scavenging compounds are those compositions
provided according to of U.S. Pat. No. 2,825,651, which is
incorporated by reference, including an oxygen remover composition
comprising an intermixing of a sulfite salt and an accelerator such
as hydrated copper sulfate, stannous chloride, or cobaltous oxide.
Another useful class of oxygen scavenging compounds are those
compositions comprising a salt of manganese, iron, cobalt or
nickel, an alkali compound, and a sulfite or deliquescent compound,
such as disclosed by U.S. Pat. No. 4,384,972, which also is
incorporated by reference.
Preferred oxygen scavenging compounds include (or include as their
base component) at least one basic iron oxide, such as a ferrous
iron oxide, or are made of mixtures of iron oxide materials. Useful
iron oxide-containing compositions are available commercially, for
example, under the "Ageless" trade name from the Mitsubishi Gas
Chemical Company of Duncan, S.C. and under the "Freshmax" trade
name from Multisorb Technologies, Inc. of Buffalo, N.Y. Also useful
are oxygen absorbing agents comprising a mixture of ferrous salts
and an oxidation modifier and/or a metallic sulfite or sulfate
compound. Such compounds react with oxygen according to the
following reaction mechanism:
It should be noted that, in the reaction scheme outlined above,
water is also consumed, an advantageous benefit in the present
application, because, as outlined previously, water is a polar
contaminant that can itself adversely affect the dielectric
properties of the vegetable oils when present in significant
quantities.
The oxygen scavenging material is encased in a housing composed
essentially of a polymeric material that exhibits a high
permeability to oxygen and that also preferably exhibits a low
permeability to water and water vapor and that exhibits significant
mechanical strength throughout the temperature range typically
encountered in electrical distribution equipment. Specifically,
useful polymeric materials are those that have an oxygen
permeability of at least about 2,000 cc-mil/100
in.sup.2.multidot.24 hrs.multidot.atm, preferably at least about
3,000 cc-mil/100 in.sup.2.multidot.24 hrs.multidot.atm, and more
preferably at least about 4,000 cc-mil/100 in.sup.2.multidot.24
hrs.multidot.atm. Permeability values for some representative
polymers are provided, for example, in G. Gruenwald, "Plastics: How
Structure Determines Properties, p. 242 (Hanser, 1992). Useful
polymeric materials also preferably have a tensile strength
measured according to ASTM method D 882 of at least about 3500 psi,
more preferably at least about 4,000 psi, and have a melting
temperature higher than about 160.degree. C.
Examples of suitable polymeric materials include polyolefins such
as high density polyethylene, polypropylene, polybutylene, and
copolymers thereof; polyphenylene oxide; polyethersulfone; nonwoven
materials, including polyester felt; and cellulose pressboards. A
particularly preferred polymer material is polymethylpentene.
The encasement housing for the oxidation reducing composition may
be made from the polymeric material in any manner that permits the
oxidation reducing composition to be in communication with the
dielectric fluid headspace and allow for the direct exposure of any
oxygen in the environment with the oxidation reducing composition.
The housing may be a simple pouch construction in which the
oxidation reducing composition is encased within a film made of the
polymeric material that is sealed to itself by ultrasonic welding,
thermal sealing or other suitable sealing method, or the housing
may be constructed of metal, hard plastic, or other suitable
material and have a "window" of film made of the oxygen permeable
polymeric material through which the oxidation reducing composition
communicates with the dielectric fluid headspace.
The encasement housing may be placed inside the electrical
distribution equipment in any configuration that allows for
communication (through the polymeric material) between the
oxidation reducing composition and the dielectric fluid headspace.
The housing thus may form an integral portion of the tank portion
of the electrical equipment that holds the dielectric fluid. The
housing may also be placed immediately inside and attached to the
dielectric fluid tank portion of the electrical equipment and held
there within the headspace of the tank.
The long term stability of the dielectric fluids of the invention
may be improved by utilizing any of the conventional methods known
for improving the stability or performance of dielectric fluids.
For example, one or more antioxidant or antimicrobial compounds may
be added to the dielectric fluid. Useful antioxidant compounds for
this purpose can be dissolved directly in the dielectric fluid
comprising the vegetable oil and include, for example, BHA
(butylated hydroanisole), BHT (butylated hydrotoluene), TBHQ
(tertiary butylhydroquinone), THBP (tetrahydrobutrophenone),
ascorbyl palmitate (rosemary oil), propyl gallate, and alpha-,
beta- or delta-tocopherol (vitamin E). It is generally also
desirable to include in the dielectric fluid one or more additives
to inhibit the growth of microorganisms. Any antimicrobial
substance that is compatible with the dielectric fluid may be
blended into the fluid. In some cases, compounds that are useful as
antioxidants also may be used as antimicrobials. It is known, for
example, that phenolic antioxidants such as BHA also exhibit some
activity against bacteria, molds, viruses and protozoa,
particularly when used with other antimicrobial substances such as
potassium sorbate, sorbic acid or monoglycerides. Vitamin E,
ascorbyl palmitate and other known compounds also are suitable for
use as antimicrobial additives to the dielectric fluid.
The performance of dielectric fluids at low temperatures is
important in some applications. Some vegetable oils do not, by
themselves, have pour point values sufficiently low to be suitable
for standard electrical power distribution applications. Vegetable
oils, unlike some conventional mineral oils, may also solidify or
gel when cooled to a temperature just slightly above their pour
point temperature for an extended period of time. A typical
electrical power distribution application requires that a coolant
have a pour point below about -20.degree. C. The dielectric fluids
of the invention, where insufficient themselves, can be modified to
ensure flowability at moderately low temperatures typically
encountered during off-cycles (lower than about -20.degree. C.).
Suitable modification of the dielectric fluids include the addition
of a pour point depressant. Suitable pour point depressants include
polyvinyl acetate oligomers and polymers and/or acrylic oligomers
and polymers.
Low temperature characteristics may also be improved by judicious
blending of oils. Certain oil blends, for example, have lower pour
points than their individual constituent oils. For example, a blend
of 25 percent by weight soya oil (I) with 75 percent by weight
rapeseed oil (II) has a pour point of -24.degree. C., compared with
-15.degree. C. and -16.degree. C. for the constituent (I) and (II)
oils respectively. Other vegetable oil blends that exhibit
similarly advantageous reductions in pour points include: 25%
soybean oil+75% oleate modified oil; 50% soybean oil+50% oleate
modified oil; and 25% soybean oil+75% sunflower oil. It will be
understood that this list of oil blends is not exhaustive and is
offered merely to illustrate the nature of the invention.
The dielectric fluids of the invention preferably are introduced
into the electrical equipment in a manner that minimizes the
exposure of the fluid to atmospheric oxygen, moisture, and other
contaminants that could adversely affect their performance. A
preferred process includes drying of the tank contents, evacuation
and substitution of air with dry nitrogen gas, filling under
partial vacuum, and immediate sealing of the tank. If the
electrical device requires a headspace between the dielectric fluid
and tank cover, after filling and sealing of the tank, the gas in
the headspace should be evacuated and substituted with an inert
gas, such as dry nitrogen, under a stable pressure of between about
2 and about 3 psig at 25.degree. C.
It is preferable in any case to minimize or eliminate the presence
of oxygen in the headspace of the electrical equipment that
contains a vegetable oil-based dielectric fluid. There are several
different approaches to the design of electrical equipment. One
design that generally is not suitable for the use of vegetable
oil-based dielectric fluids is the conservator non-sealed type. A
more common design type in ANSI/IEEE standard electrical
distribution and medium power equipment employs the use of a tank
headspace to allow for the expansion and contraction of the tank
contents. Even if the headspace of the equipment is purged of air
and replaced with inert gases, it is possible over the operating
life for oxygen (air) to leak into the headspace from the openings
of the cover or accessories, the slow migration of air through
gaskets, and the operation of pressure relief devices. Ingress of
oxygen into the headspace will eventually contribute to the
consumption of any antioxidant additives in the fluid. It is also
desirable therefore to minimize the presence of oxygen throughout
the lifetime of the electrical equipment through careful design and
manufacture. One such method for reducing the ingress of oxygen
into the dielectric tank is to weld any components, covers, or
access points that communicate with the tank headspace, as gaskets
and other means for sealing such openings are all susceptible to
leakage over time.
The dielectric fluids of the invention may be used in any
application into which conventional dielectric fluids are employed.
Thus, the vegetable oil based fluids of the invention may be
incorporated into all types of electrical equipment, including, but
not limited to, reactors, switchgear, regulators, tap changer
compartments, high voltage bushings, and oil-filled cables.
Cables that are used for the transmission and distribution of
electricity generally incorporate a dielectric fluid, and are often
referred to simply as oil-filled cables. Oil-filled cables
typically comprise at least one conductor around which there is
provided a solid, stratified insulation formed by windings of
insulation material tapes that are impregnated with an insulating
oil. These cables also generally have at least one longitudinal
duct or canal that allows for the movement of the insulating oil
along the length of the cable. Oil-filled cables are used both for
underwater and land-based applications, and particularly where
submerged underwater, filled cables can be extremely sensitive to
gasing tendency. Because they are most often pressurized, leakage
from oil-filled cables can have a greater environmental impact from
release of insulating fluid. The dielectric fluids of the present
invention can be used to fill electrical cables. They can also be
used to retrofill cables that initially contain a non-vegetable oil
dielectric fluid.
Electrical transformers and switchgear typically are constructed by
immersing the core and windings and other electrical equipment in a
dielectric fluid and enclosing the immersed components in a sealed
housing or tank. The windings in larger equipment frequently are
also wrapped with a cellulose or paper material. The dielectric
fluid of the invention can be used to fill new electrical equipment
in the manner described above. The fluids can also be used to
retrofill existing electrical equipment that incorporate other,
less desirable dielectric fluids. Retrofilling existing equipment
can be accomplished using any suitable method known in the art,
though because of the increased sensitivity of vegetable oil fluids
to moisture, it is important first to dry components of the
electrical equipment prior to the introduction of the vegetable oil
based dielectric fluid. This is important especially with respect
to the cellulose or paper wrapping, which can absorb moisture over
time. Because of the relatively high solubility of water in
vegetable oils, a vegetable oil fluid can itself be used to dry out
existing electrical equipment.
One method of retrofilling mineral oil containing transformers is
discussed generally by Sundin in Retrofilling Mineral Oil
Transformers With Fire Resistant Fluids, Electricity Today, pp.
14-15 (May 1996), which is incorporated by reference. A useful
method for retrofilling oil-filled electrical transmission cables
is described in U.S. Pat. No. 4,580,002, which is incorporated by
reference. Other suitable methods will be known by those skilled in
the art.
The following descriptions and Figures are offered to provide an
illustration of the invention and are given in reference to an
electrical transformer. It will be understood by those skilled in
the art, however, that the compositions and methods encompassed by
the present invention are equally suited for use in all types of
electrical equipment, including those described above. These
descriptions are to be understood as preferred and/or illustrative
embodiments of the present invention and are not intended to limit
the scope thereof.
Referring now to FIG. 1, a transformer tank 10 typically comprises
a tank body 12, a tank cover 14 bolted or welded to tank body 12
and sealed with gasket 16. Tank body 12 is sealed. Tank 10 houses
the transformer core and windings (not shown) or other electrical
equipment, immersed in a dielectric fluid 18. The space between the
surface of the fluid and the tank cover is the tank headspace 20.
According to one embodiment of the present invention, a polymer
container 22 containing an oxidation reducing composition is
mounted in the headspace of the tank, preferably on the inside of
the tank cover as shown in FIG. 1. As set forth above, container 22
is a pouch or bag encasement constructed of a oxygen permeable
film.
A simple embodiment of the oxidation reducing composition is shown
in FIG. 2 where a pre-packaged oxygen scavenging compound 8, such
as is available commercially under the Ageless and Freshmax trade
names, is encased in a "pouch" 22 constructed of a oxygen permeable
polymer film, a polyester felt or a cellulose pressboard. The pouch
is attached to the tank cover 14 by means of a simple clasp 6 or
other suitable fastening means. This embodiment finds particular
utility in relatively small electrical equipment, such as in
pole-mounted transformer assemblies.
According to another preferred embodiment shown in FIG. 3, the
container 22 is supported in a polyolefin housing 24 mounted
adjacent to a threaded opening 26 in the tank cover. A threaded
plug 28 seals the container in the opening in the tank cover 14 and
preferably includes a transparent view port 30. It will be
understood that view port 30 can alternatively be incorporated into
another part of the tank cover or tank wall.
When it is desired or necessary to replace the container containing
the oxidation reducing composition, the threaded plug 28 can be
removed, and the container 22 removed from the polyolefin housing
and replaced. The low gas permeability of housing 24 prevents
significant gas exchange between the headspace 20 and the outside
atmosphere during the short period of time that the threaded plug
is removed. This can be accomplished even though the gas
permeability of the container is not so high as to impede the
operation of the oxidation reducing composition over more extended
periods of time.
Still in reference to FIG. 3, in addition to the oxidation reducing
composition, it is preferred to provide a means for indicating the
presence of oxygen in the tank headspace. This indicator preferably
is an oxygen sensitive compound 32 such as that marketed by
Mitsubishi Gas Chemical Company under the trade name Ageless Eye.
This compound exhibits a pink-to-blue color change when the ambient
oxygen concentration exceeds 0.1%.
The oxygen indicator preferably is housed in the tank headspace
wall in such a manner that it can both chemically contact the gas
in the headspace and be visible for inspection from the outside of
the tank. One way to accomplish this is to mount the oxygen
indicator adjacent to the view port 30 as shown.
In addition to the foregoing, the use of a vegetable oil-based
dielectric fluid in transformers can be facilitated through several
modifications to the transformer tank. These include providing a
sealed, accessible chamber such as described above in which the
oxidation reducing composition can be replaced without increasing
the exposure of the fluid in the tank to outside air. Other
modifications reduce the leakage of the gas from within the tank,
to thereby reduce the long-term exposure of the fluid to air.
Referring to FIG. 4, one such modification relates to the volume of
the tank headspace 20. Current ANSI/IEEE C57 series standards, for
example, require distribution transformer tanks to remain sealed
over a top oil temperature range of from -5.degree. C. to
105.degree. C. for pole-mounted and pad-mounted designs and over a
100.degree. C. top oil range for substation transformers. Outside
this range the tank is typically vented to avoid damage to the tank
or related equipment. According to the present invention, the
headspace volume is increased so that the temperature range over
which the tank remains sealed increases correspondingly, thus
reducing the probability of oxygen (air) leaking into the tank.
Specifically, the present tank preferably includes a headspace
volume sufficient to allow the tank to remain sealed from
-20.degree. C. to 115.degree. C.
In addition, each tank includes an automatic pressure release
device (PRD) 40 for venting the tank as described above. The PRD 40
can be calibrated to automatically vent headspace gas when the
internal pressure exceeds acceptable levels, typically 9.+-.1 psig,
and to automatically reseal when the pressure reduces to a desired
level, typically to 6.+-.1 psig. Because the PRD reseals at a
positive pressure, the headspace will maintain a positive pressure
even after venting by the PRD. Maintaining a positive pressure in
the headspace helps to prevent the ingress of air into the
tank.
It is also preferable to replace conventional gaskets (not shown)
with gaskets made from a material that is substantially gas
impermeable. It will be understood that such gasket material must
also be resistant to degradation by the dielectric fluid. Examples
of suitable gasket material include nitrile rubber with a high
acrylonitrile content, and various fluoroelastomers, of which the
compound sold under the trade name VITON from the E.I. du Pont
Nemours & Company, is representative. Other suitable
fluoroelastomers are available commercially from Dyneon LLC of
Oakdale, Minn. Materials with a relatively high gas permeability,
such as silicone rubber and nitrile rubber having a low
acrylonitrile content, are less suitable for gasket material. It
will be understood that this list is illustrative only, and that
other resilient, gas impermeable materials could be used to form
the gaskets for the transformer tank. As mentioned above, another
way to avoid the leakage associated with the long-term use of
gaskets is to weld the equipment housing shut and thereby eliminate
completely gasketed seals.
Another method for reducing gas ingress is to reduce or eliminate
altogether the headspace and provide for thermal expansion by other
means. The pressure/partial vacuum withstand would be based on a
thermal range of the average fluid temperature of about -20.degree.
C. through about 115.degree. C.
For units with sufficient headspace, vegetable oil-based dielectric
fluids could also serve as excellent material in the recent
development of High Temperature Transformers, which typically have
a maximum top oil rated temperature rise over ambient of
115.degree. C.
In addition to the foregoing, vegetable oil-based dielectric fluids
in electrical equipment in which paper insulation has been
substituted by non-cellulose insulating "paper" would have greater
inherent stability. This is largely because cellulose materials
liberate water as they degrade thermally. Candidate materials
include aramid insulating materials, polyester materials, and
polyamides.
While preferred embodiments of the invention have been shown and
described hereinabove, modifications thereof can be made by one
skilled in the art without departing from the spirit and scope of
the invention.
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